1. Field of the Invention
The present invention relates to a light-emitting element excellent in emission intensity. In particular, the present invention relates to an LED element using an excitation source of light in the near UV to blue region, and it also relates to a fluorescent substance used therein.
2. Background Art
As for fluorescent substances of rare earth metals coordinated by organic ligands such as phosphine oxide and β-diketonato ligands and for LED elements employing them, concepts and examples thereof are described in, for example, Japanese Patent No. 3811142. Organic fluorescent substances like them are superior to inorganic ones in solubility and resin-dispersability in solvents, and hence by utilizing this character the light-extraction efficiency is increased to obtain strong emission intensity in the above examples. In the case where an LED or LD is adopted as an energy source of luminescence, light in a particular wavelength range is used. In many conventional cases, the maximum excitation wavelength, which gives the strongest emission intensity, is often positioned in the wavelength range of 300 nm to 350 nm. Actually, however, since there is a fear that light in the above wavelength range gives harmful effects to the human body, light at a longer wavelength is generally used in practice as the excitation light. It is, therefore, difficult to use light at the most effective wavelength and accordingly it is inevitable to use fluorescent substances of low emission intensity. On the other hand, if the skeletons of ligands are changed so that the fluorescent substances can absorb light at a longer wavelength, the efficiency of energy transfer between the metal and the ligands is so lowered that practically satisfying emission intensity cannot be obtained. Thus, this is a dilemma.
Rare earth metals have specific energy levels of f-electrons, and the electrons of rare earth metal can be excited by irradiation of light having an energy corresponding to the energy level gap between the vacant orbital and the occupied orbital. The luminescence induced by this excitation is not accompanied by the energy transfer from the ligands, and hence does not suffer loss of the energy transfer efficiency. Accordingly, the luminescence generally results from the internal energy conversion with high efficiency. This means that the emission from the excited state occurs with high efficiency. The wavelength corresponding to the energy level gap of f-f transition is often much longer than 400 nm. For example, the transition of 5D0→7D2 in Eu corresponds to 465 nm. The f-f transition of rare earth metal can be, therefore, excited by light at a long wavelength. This means that near ultraviolet LEDs, which emit light around 400 nm, and blue LEDs, which emit light around 460 nm, can be used as an excitation source.
From the viewpoint of quantum chemistry and group theory, the f-f transition of rare earth is forbidden. The forbidden transition is generally activated when the symmetry of orbital is broken in the metal atom. However, the orbital in broken symmetry deviates from the most stable state, and hence is too unstable in energy to break the symmetry drastically and to keep the broken symmetry. In some normally present compounds, the f-f transition is sometimes slightly allowed because of distortion induced by asymmetry of coordination environment around the metal atom. The f-f transition thus has activity, but the activity is nevertheless generally not so large that the excitation occurs with high possibility. Accordingly, even if the emission from the excited state proceeds with high efficiency, the total emission efficiency based on the applied light is often smaller than that of the luminescence by way of the energy transfer from the β-diketonato ligand. For this reason, the luminescence excited by f-f transition has not been satisfying enough to use in LEDs.
Journal of Alloys and Compounds 408-412, pp 921-925 (2006) describes that a rare earth fluorescent substance containing a β-diketonato ligand emits stronger luminescence when also coordinated by a phosphine oxide ligand than when not coordinated by the phosphine oxide ligand. Further, JP-A-2005-252250 (KOKAI) discloses that a fluorescent substance comprising both β-diketonato and phosphine oxide ligands emits strong luminescence when excited at a wavelength in 380 nm to 410 nm. However, even the fluorescent substances described in the above publications exhibit weak emission when excited at a wavelength around 465 nm, where the excitation band of f-f transition in Eu3+ is positioned, and hence the f-f transition is not fully activated.
JP-A-2007-46021 (KOKAI) discloses a compound of a rare earth metal coordinated by a ligand of phosphate triester, and it also discloses a luminous composition comprising a resin such as silicone resin and the rare earth complex compound dispersed therein. The disclosed substances emit strong luminescence assigned to an f-f transition observed in the excitation spectrum. However, there is a problem that phosphate triesters are liable to react with fluorine compounds to form fluorophosphate compounds, which are generally fatally poisonous. In addition, the rare earth complex emitting strong luminescence contains a β-diketonato ligand having a fluoroalkyl group such as CF3, and factories treating silicon compounds such as silicone resin also often treat fluorine compounds such as hydrofluoric acid. Accordingly, the disclosed composition is far from practical in view of safety.
Journal of Luminescence 117, pp 163-169 (2006) describes a composition in which a rare earth metal complex is combined with the skeleton of Si—O—Si bonds by the sol-gel method. However, it is also reported that the f-f transition was not observed in the excitation spectrum monitored at 612.5 nm. Accordingly, it has not been known that the f-f transition is activated when the fluorescent substance is combined with the skeleton of Si—O—Si bonds.